the center of a womans breast that become pointed, hard, and erect when they are stimulated or the woman is horny...or really cold...
not a sexual organ, but generally feels good when rubbed, sucked, licked, bitten, or vibrated.it typically turns guys and girls on when they see a girls hard nipples showing through her shirt, especially when the girl isn't wearing a bra.

seeing two hot girls with big tits rub their hard nipples together is enough to get any guy or girl off.